1. Field of the Invention
This invention pertains generally to aerodynamic load control devices, and more particular to a translational device for controlling lift of an airfoil.
2. Description of Related Art
Aerodynamic load control devices are common on lifting surfaces on airplanes, rotorcraft, wind turbines and other lift generating systems. In an airplane, an aileron is a typical aerodynamic control device used to change the lift and drag properties of the airfoil. Despite their lift enhancement properties, however, conventional aileron devices tend to be bulky and heavy and often require complex systems for actuation and support. Also, these devices are prone to flutter and as a result require special attention in the design and development stage. In addition, conventional devices tend to require intensive and costly maintenance throughout the lifespan of the system.
A conventional control surface or simple flap is a separate moveable lifting surface that typically occupies the aft 20% to 30% of the chord of a lifting surface.
As illustrated in FIG. 1, in a conventional airfoil 10, rotating the control surface or flap 12 about its hinge point 14 results in a change in surface camber which in turn causes a change in the circulation of the air flow 16 and, thus, the lift 18 of the entire lifting surface. For example, raising flap 12 to position 20 will cause a decrease in lift, while lowering flap 12 to position 22 will cause an increase in lift. It is well known that the optimum location for subsonic lift control in aircraft is at the trailing edge of an airfoil since small changes in the flow field near the trailing edge can result in large changes in the overall flow field. The trailing-edge geometry of a lifting airfoil or surface has a significant influence on the aerodynamic performance of the airfoil at subsonic and transonic flow conditions.
One example of small changes in the flow field near the trailing edge creating large changes in the overall flow field is the trailing-edge blowing concept. Here, large increases in lift are obtainable when tangential surface blowing occurs over a rounded trailing edge. This pneumatic concept can greatly simplify high-lift system complexity and also replace the control surfaces on aircraft. The major problems with this concept are 1) the complexity, weight, and cost associated with the piping of substantial amounts of high-pressure air, (2) the increase in engine size and, hence, weight and cost, necessitated by the loss in engine mass flow for the pneumatic system, or the need for pumps (many small ones or one or two large ones) to generate the required mass flow, and (3) the problem of making this concept reliable and failsafe; i.e., a loss in engine power or an engine failure should not result in a loss of airplane control.
Instead of trailing-edge blowing, it may be easier to deploy a small trailing-edge flap for lift control. An example of such a device is a “Gurney-flap” which consists of a small (approximately 0.01×airfoil-chord), fixed vertical tab mounted perpendicular to the lower (pressure) surface at the trailing edge. FIG. 2 shows the relationship between the coefficient of lift, CL, and angle of attack, α, for a 0.125c Gurney-flap in comparison to a clean airfoil. While Gurney-flaps enhance lift in the linear range as shown in FIG. 2, they may also cause a significant drag penalty especially at low lift conditions, such as cruise flight. This drag penalty is the main reason why Gurney-flaps are used on only a few aircraft configurations for which high maximum lift is more important than low cruise drag. To avoid the drag penalty, miniature split flaps hinged to the airfoil lower surface have been conceptualized. While these split flaps would be stowed during cruise so as to eliminate drag, their implementation has been hampered by the fact that the aft portion of an airfoil with a sharp trailing edge does not provide sufficient structural support or volume for hinges and deployment hardware based on conventional manufacturing technology.